Reservoir treatments

ABSTRACT

The present invention relates to the field of enhanced oil recovery and provides a method of establishing a plug in a hydrocarbon reservoir, the method comprising introducing into the reservoir a formulation comprising solid particles and a viscosifier and then reducing the viscosity of said viscosifier, thereby causing said solid particles to form a plug within said hydrocarbon reservoir. Also provided is a method of establishing a plug in a hydrocarbon reservoir, the method comprising introducing into the reservoir a formulation comprising:
         (a) microorganisms or cell-free enzymes;   (b) solid particles; and   (c) a viscosifier which is a substrate for the microorganisms or cell-free enzymes of (a).       

     Also provided is a formulation comprising:
     (a) microorganisms or cell-free enzymes;   (b) solid particles made from wood or a wood derived product; and   (c) a viscosifier which is a substrate for the microorganisms or cell-free enzymes of (a).

The present invention relates to a method of treating a hydrocarbonreservoir in order to prepare it for extraction of hydrocarbonstherefrom and to formulations for use in such methods. Moreparticularly, the methods relate to plugging of a hydrocarbon reservoirin order to alter the flow of liquid through the reservoir.

Many hydrocarbon reservoirs have a primary production phase where aproduction well drilled into the formation results in travel of oil tothe surface; the reservoir drives come from natural mechanisms,optionally enhanced by pumps. Later in the working life of a reservoirpressure will fall and a liquid, often water, may be pumped via aninjection well into the formation to force the hydrocarbon, usually oil,through the formation and into the production well. This is thesecondary recovery phase. Over time, certain reservoirs, in particularcarbonate reservoirs, will experience a steep decline in production asoil found within the matrix of the formation is not readily flushed out.The injection liquid will follow a path where the flow resistance is atits least, a flow channel, also referred to as a connected fracturesystem or water thief zone. In the case of a carbonate reservoir thiswill typically be a fracture within the matrix formed as a result ofchalk erosion. Erosion may be chemical or physical. Erosion is increasedby the pressure from liquid injection and increased flow rate throughthe matrix. As a consequence water penetration of the matrix is limited.

Carbonate rocks (chalk and limestone) account for more than half of theworld's hydrocarbon reservoirs. Carbonate reservoirs typically comprisea matrix, which provides the main oil storage capacity, and havefractures within the matrix. There is a need for improved methods ofrecovering oil from carbonate reservoirs.

It is possible to calculate or model how the injection liquid flows inthe reservoir. For example, it is known for low-level radioisotopes withrelatively short half-lives to be added as tracers to the injectionwater. Radiation from the isotopes can be identified in the productionwell. It is thus possible to estimate the time it takes for theinjection liquid to pass from the injection well to the production well.Alternatively, specific chemicals, for example nitrate, can be used astrace substance.

The ability of the injection liquid to force the oil forwards isreferred to in the field as the sweep efficiency of the injectionliquid. Water thief zones caused by dominant fractures between injectionand producer holes acting as flow channels will reduce the sweepefficiency (FIG. 1). It is known that the sweep efficiency of theinjection liquid can be improved if a plug is formed in the fracturesystem. The plug can be partially permeable, but the flow resistanceincreases such that the injection liquid is forced to flow around theplug and thus into those parts of the reservoir that now have the leastflow resistance. The sweep efficiency of the injection liquid isimproved in this way. Ideally the sweep efficiency as compared to theoriginal sweep efficiency will be enhanced but a return to the originalsweep efficiency will still result in improved oil recovery. Plugs inthe middle third (FIG. 1) may improve the sweep efficiency beyond thatof the original sweep efficiency.

Plugs can be produced by admixing gel-forming, water-soluble polymers tothe injection water. The polymers can be synthetic, for examplepolyacrylamide, or biological. Xanthan, for example, is used as abiopolymer and is discussed inter alia in patent documents U.S. Pat.Nos. 4,716,966, 4,485,020, 4,947,932, and GB 2,246,586. Patent documentU.S. Pat. No. 5,028,344 discusses the use of cellulose and modifiedcellulose, while patent document U.S. Pat. No. 5,010,954 discusses theuse of guar gum and carboxymethylcellulose.

It is disclosed in WO 2012/164285 how a plug may be generated whichrelies on microbial biomass, such a ‘living plug’ offers benefits interms of plug control. However, it requires a continuous supply ofnutrients which may be costly; in some circumstances, a static plugwhich does not require a continuous supply of nutrients would bedesirable.

When attempting to plug a reservoir it is especially difficult to ensurethat the right zone is plugged; in particular there is a need to providea plug which can travel through the reservoir to a target position. Itwould be undesirable if the plug introduced into the reservoir had sucha high tendency to plug that it blocked fractures adjacent to theinjection point. On the other hand, a plug must be able to withstandsignificant pressures if it is to stand firm and force water into thesurrounding matrix. Prior to the present invention, no plugging systemshave been described which enable plugging remotely from the injectionwell, i.e. deep into the reservoir.

The present inventor has developed a plugging system that addressesthese needs and in one aspect the present invention provides aformulation comprising:

(a) microorganisms (including those which are mesophiles, thermophiles,extreme thermophiles or hyperthermophiles);(b) solid particles, preferably made from wood or other cellulosicmaterial;(c) a viscosifier which is a substrate for the microorganisms of (a);and optionally(d) growth medium.

In preferred embodiments is provided a formulation comprising:

(a) microorganisms which are:

-   -   (i) mesophiles, thermophiles, extreme thermophiles or        hyperthermophiles,    -   (ii) unable to utilise hydrocarbons as a carbon source,    -   (iii) not indigenous to the hydrocarbon reservoir, and        optionally    -   (iv) cellulolytic or hemicellulolytic;        (b) solid particles, preferably made from wood or other        cellulosic material;        (c) a viscosifier which is a substrate for the microorganisms of        (a); and optionally        (d) growth medium.

Thermophiles, extreme thermophiles or hyperthermophiles are generallypreferred.

The formulation is introduced into the reservoir in the form of a liquidsuspension which is mobile within the flow channels of the reservoir. Asa result of the interactions of the components within the formulation,the suspension converts into a mass which can effectively plug one ormore fractures or channels within the reservoir. This transition frommobile liquid to pluggable mass is achieved because the microorganismsdegrade the viscosifier which had served to reduce the overall frictionof the formulation, allowing it to pass as a cohesive and mobile slugthrough the reservoir. The viscosifier had enabled the suspension toflow through the reservoir, when this is removed the particles can bepacked by back pressure and press against the walls of the flow channel,expelling liquid from within the formulation and causing a plug mass.Total friction of the particles against each other and against the wallswithstands the back pressure; the longer the plug the greater thefriction and the greater the forces which can be withstood.

The formulation of the invention is intended for introduction into ahydrocarbon reservoir and is provided in suitable containers, e.g. acontainer with a capacity of at least 25 litres, preferably at least 100litres, more preferably at least 500 or 1000 litres. Thus theformulation will typically be provided in these volumes, or ready to begenerated in these volumes on addition of water to the desiredviscosity.

Each injection of the formulation of the invention into the reservoirwill be of around 10-100 m³, preferably 30-70 m³.

The microorganisms may be capable of sporulation and it may be preferredto include them as spores in the formulation which is introduced intothe reservoir.

The microorganisms are preferably unable to utilise hydrocarbons (i.e.downhole oil and gas reserves) as a carbon source, i.e. preferablycannot support their own growth exclusively in hydrocarbons as a sourceof carbon. However, as shown in the Examples, while the microorganismswill typically not grow within the oil, the presence of oil may not betoxic. In general, the microorganisms do not thrive outside theenvironment of the slug.

For performance of the invention there is an essential relationshipbetween the microorganisms and viscosifier, namely that themicroorganisms can degrade the viscosifier and, preferably, theviscosifier can act as sole or primary carbon source for themicroorganisms. As discussed in more detail below, the viscosifier alsoserves to reduce friction and acts as a carrier, allowing the wholeformulation to move through flow channels in the reservoir.

The microorganisms are able to degrade the viscosifier and arepreferably saccharolytic (e.g. polysaccharolytic) or lignocellulolytic,more preferably cellulolytic or hemicellulolytic.

Thus, the viscosifier is preferably a polysaccharide or derivativethereof or derived from lignin. The viscosifier will typically be apolymer in order that it has the physical properties to perform itscarrier function. A low molecular weight viscosifier such as glycerolmay be used together with a high molecular weight degradable polymer.The primary polymeric viscosifiers typically have a weight averagemolecular weight of 50,000-500,000.

The viscosifier will typically have a high molecular weight so that itcan provide high viscosity and low friction (acting as a lubricant)until it is degraded. In addition to a high molecular weight component,a further viscosifying substrate may be used, e.g. glycerol.

Cellulolytic and hemicellulolytic microorganisms are able to grow oncellulose and/or hemicellulose. Most cellulolytic organisms are alsohemicellulolytic. Preferably the microorganisms are able to utilisecellulose and/or hemicellulose as sole carbon source. In practice theycan also use other carbon sources, in particular derivatives anddegradation products of these complex polysaccharides, such ascarboxymethyl cellulose (CMC). ‘Hemicellulose’ encompasses a widevariety of hetero-polysaccharides, the polysaccharides are typicallybranched and amorphous and may comprise many different sugar monomers,e.g. xylose, mannose, galactose, arabinose. Cellulose, in contrast,consists only of glucose monomers.

The microorganisms are preferably bacteria. Preferred bacteria accordingto the invention include the cellulolytic bacteria Clostridiumthermocellum and Acidothermus cellulolyticus. Different reservoirs areat different temperatures and the microorganisms can be selectedaccordingly; A. cellulolyticus thrives at higher temperatures than C.thermocellum and so is more suitable for hotter reservoirs. C.thermocellum is especially preferred and the strain known as JW20 (ATCC31549) is most preferred. Further suitable bacteria are described bySissons et al. in Applied and Environmental Microbiology, April 1987, p832-838, in particular the strain designated TP8.T and deposited underthe name Caldicellulosiruptor saccharolyticus (ATCC 43494).

Further suitable cellulolytic bacteria include: Caldocellulosiruptorsaccharolyticus, Caldocellulosiruptor lactoaceticus,Caldocellulosiruptor kristjanssonii, Anaerocellum thermophilium,Butyrivibrio fibrisolvens, Ruminococcus flavefaciens, Ruminococcussuccinogenes, Ruminococcus albus, Eubacterium cellulolyticum,Clostridium acetobutylicum, Clostridium chartatabidum, Clostridiumcellulovorans, Clostridium herbivorans, Clostridium cellulosi,Clostridium cellobioparum, Clostridium papyrosolvens, Clostridium josui,Clostridium cellulolyticum, Clostridium aldrichii, Clostridiumstercorarium, Clostridium thermocellum, Clostridium cellulofermentans,Clostridium celerescens, Clostridium thermopapyrolyticum, Clostridiumthermocopriae, Clostridium sp. C7, Bacteroides sp. P-1, Bacteroidescellulosolvens, Acetivibrio cellulolyticus, Acetivibrio cellulosolvens,Thermoactinomyces sp. YX, Caldibacillus cellulovorans, Bacilluscirculans, Acidothermus cellulolyticus, Cellulomonas biazotea,Cellulomonas cartae, Cellulomonas cellasea, Cellulomonas cellulans,Cellulomonas fimi, Cellulomonas flavigena, Cellulomonas gelida,Cellulomonas iranensis, Cellulomonas persica, Cellulomonas uda,Curtobacterium falcumfaciens, Micromonospora melonosporea, Actinoplanesaurantiaca, Streptomyces reticuli, Streptomyces alboguseolus,Streptomyces aureofaciens, Streptomyces cellulolyticus, Streptomycesflavogriseus, Streptomyces lividans, Streptomyces nitrosporeus,Streptomyces olivochromogenes, Streptomyces rochei, Streptomycesthermovulgaris, Streptomyces viridosporus, Thermobifida alba,Thermobifida fusca (Thermomonospora), Thermobifida cellulolytica,Thermomonospora curvata, Microbispora bispora, Fibrobacter succinogenes,Sporocytophaga myxococcoides, Cytophaga sp., Flavobacterium johnsoniae,Achromobacter piechaudii, Xanthomonas sp., Cellvibrio vulgaris/fulvus,Cellvibrio gilvus, Cellvibrio mixtus, Pseudomonas fluorescens(cellulose), Pseudomonas mendocina, Myxobacter sp. AL-1.

The microorganisms can conveniently be grown in fermenters. It may bedesired to transport and/or inject the bacteria in spore form. Keepingthe microorganisms cool and/or manipulating pH can maintain them inspore form.

The particles are preferably made from solid wood, e.g. Corylus,preferably Corylus avellana (hazel), Pinus (pine), Betula (birch) andQuercus (oak). Wood is the lignocellulosic material found between thepith and bark of a tree or shrub. Alternatively the particles may bemade from a wood derived product, e.g. hardboard, particle board, MDFetc. Such products typically comprise wood fragments and a polymericbinder such as a resin, the products generally being produced at hightemperatures and under pressure.

The particles are able, when packed together, to form a plug in afracture which is sufficiently resistant to pressure generated byinjection liquid that the plug can force injection liquid into thematrix around the fracture or channel. Thus solid particles according tothe present invention when present in water or in another low viscosityenvironment can form a plug which interrupts liquid flow through ahydrocarbon reservoir.

The particles may be made from non-cellulose based polymeric materials,both naturally occurring and synthetic or chemical crystals. Thefunctional requirements of the particles are described herein andsuitable materials can be selected to deliver these properties.Deformable polymeric particles may be made, for example, frompolyacrylamide. Such particles may be nanoparticles.

A further type of suitable solid particles are so called Ugelstadparticles, produced by their Method #2, or more preferably by theirMethod #3, the ‘Ugelstad Process’, a two-step swelling process based onpolymer seed particles. The seed particles are activated (swollen) by asolvent which is then removed allowing high levels of monomer uptake.This process is very flexible and allows the size, density anddeformability of the particles to be manipulated in order to achieveparticles of the type desired for use according to the present invention(Ugelstad et al. Adv. Colloid Interface Sci. 13, 101 (1980); Ugelstad etal. J. Polym. Sci 72, 225 (1985) and Ugelstad et al Makromol. Chem.Suppl. 10/11, 215 (1985)). These particles are generally 1-100 μm indiameter and may have a standard deviation in diameter of around 1%(i.e. less than 2%).

The formulation will typically contain 25-70% particles, by volume,preferably 35-50%, more preferably 40-45%.

The particles are preferably 0.05 to 4 or 5 mm in diameter, morepreferably 0.2 to 1 mm in diameter. Although they may be smaller, e.g.nanoparticles, which may have a diameter of 50 to 10,000 nm e.g. 100 to1000 nm. The preferred diameter of the particle is dependent on thediameter (width) of the fracture to be plugged, for example a fractureof 13 mm is well plugged by particles 0.5 mm in diameter. Tracers can beused to give information about fracture width in a matrix. The particlesin any given formulation will usually be substantially uniform, in sofar as the production method allows. The particles are preferablysubstantially spherical when not compressed. Particles of 0.2 to 0.5 mmin diameter will be preferred in some applications.

The particles of the invention are preferably deformable under pressure(as opposed, for example, to rock particles which are not deformable),this assists in their ability to pack together and to plug the reservoirthrough forces exerted through the packed particles and against thewalls of the fracture. In particular, particles greater than 0.1 mm indiameter will generally be deformable. The particle based plug may leakwater until deformation into an oval shape and packing results in asubstantially complete seal of the fracture/channel. Thus, “deformable”will be understood in the context of the invention in terms of theability to form a plug down-hole when packed and to withstand the kindsof pressures disclosed herein. The particles may have a core which ishard, essentially not deformable, but the outer layer is deformable,e.g. the outer 10-30%, e.g. 20%. As described herein, that mayconveniently be achieved by pre-softening of wooden particles or by alayered design to the production of Ugelstad particles.

As described elsewhere herein, the particles are generally substantiallyuniform in size and are typically about as high as they are wide, sospherical or substantially spherical or cuboid, e.g. cuboid with roundedcorners, preferably spherical or substantially spherical. ‘Diameter’will be understood with these particle shapes in mind.

It may be desirable, for example when plugging fractures greater than 1mm, in particular greater than 2 or 3 mm wide, to use two differenttypes of solid particles. The first type have a diameter greater than0.05 mm, typically greater than 0.1 or 0.2 mm, and are deformable.Preferably they have a density similar to that of water (fresh water orsea water—salinity 35 g/L), e.g. a density which is 80-120% of thedensity of pure water, preferably 90-110%. Such particles are preferablymade of wood and have the associated characteristics described elsewhereherein. As described elsewhere herein, the particles are generallysubstantially uniform in size and are typically about as high as theyare wide, so spherical or substantially spherical or cuboid, e.g. cuboidwith rounded corners, preferably spherical or substantially spherical.The particles are typically less than 5 mm, preferably less than 3 mm indiameter.

The second type of particle, which may be introduced into the reservoirafter the first type, but which are preferably introduced at the sametime, are smaller. They are at least four times smaller than the firstparticle type, preferably at least 5 times smaller than the firstparticle type, e.g. 5-30 or 10-20 times smaller. These particles may bedeformable but need not be, so sand may be used for these particles.Preferably they have a density similar to that of water (fresh water orsea water—salinity 35 g/L), e.g. a density which is 80-120% of thedensity of pure water, preferably 90-110%. Again, preferably theparticles are generally substantially uniform in size and are typicallyabout as high as they are wide, so spherical or substantially sphericalor cuboid, e.g. cuboid with rounded corners, preferably spherical orsubstantially spherical. These particles are typically smaller than 0.2mm, preferably smaller than 0.05 mm, most preferably smaller than 0.025mm in diameter and may preferably be Ugelstad particles. These particlesare typically greater than 1 μm in diameter.

Alternatively a single population of small particles may be used to forma plug. These are smaller than 0.1 mm, preferably smaller than 0.05 mm,e.g. 1 μm to 50 μm in diameter. These particles are preferablydeformable but need not be. The particles are generally substantiallyuniform in size and are typically about as high as they are wide, sospherical or substantially spherical or cuboid, e.g. cuboid with roundedcorners, preferably spherical or substantially spherical. Preferablythese are Ugelstad particles.

Alternatively, a single population of very small solid particles may beemployed in the form of a colloid, typically these particles are 1-1000nm in diameter but possibly are even smaller. The particles aredispersed within the viscosifier (which is the continuous phase).Degradation of the viscosifier causes packing of the dispersed solidparticles and the formation of a plug, as described elsewhere herein.Colloids may be particularly suitable when plugging in sandstonereservoirs.

A population or “type” of solid particles consists of substantiallyidentical particles and thus within a formulation a first population ofparticles of one size may be provided together with a second populationwhich is at least 3, preferably at least 5 times larger. Size isgenerally considered in terms of diameter. A formulation may containmore than two populations of solid particles but will typically compriseonly one or two populations.

In carbonate reservoirs fractures exist of varying width. With knowledgeof the dimensions of the fracture, the length of the plug which it isintended to make can be estimated and a suitable volume of theformulation of the invention injected into the reservoir. The pluggingis essentially achieved because of friction between the particles andthe walls and between the particles themselves; the inventor has foundthat a longer plug (in the flooding direction) is able to withstandgreater pressures.

The present invention is also suitable for use in plugging sandstonereservoirs and more specifically the water channels found therein andthe same principles apply. Connected pore channels within the sandstonemay be blocked to form a plug. Also, there may be microscopic fracturesclose to the injection well, blocking of these fractures will improvesweep efficiency.

As described in more detail in the Examples, the particles may begenerated by standard slurrification processes. This may result insaturation of the particles, which is desirable to equalize theirbuoyancy with water, as the reservoir environment is largely aqueous.The slurrification process also allows for particles of a homogenous andknown size to be generated, in particular to control the maximumdiameter of the particles.

Particles of the invention are preferably soakable so that they can havea density approximately (e.g. within 10%, preferably within 5% of thedensity of pure water) the same as water after they have been submerged.Alternatively they are manufactured to have a density approximately(e.g. within 10%, preferably within 5% of the density of pure water) thesame as water if they are not able to absorb water. The formulation ispreferably aqueous and the particles are preferably saturated. Dependingon whether salt or fresh water is used in the formulation the density ofthe particles can be adjusted slightly to allow for the higher densityof salt/sea water.

More generally the buoyancy/density of the particles is similar to thatof the water (fresh water or sea water, salinity 35 g/L) e.g. a densitywhich is 80-120% of the density of pure water, preferably 90-110%. Theformulation is aqueous and it is desirable to have approximateequilibrium in density between the solid particles and the rest of theformulation (which is largely viscosifier plus water) so that theparticles neither float to the top of the formulation or sink to thebottom. Equalization avoids that and makes flow through the reservoireffective, even at lower flow rates (e.g. at about 1 m per daydown-hole).

The particles preferably do not have a smooth surface, this enhancestheir plugging capabilities. Without wishing to be bound by theory, thisenhanced plugging may be due to increased friction of the particles. Insome preferred embodiments, the particles have a surface withmicroscopic hair-like structures. This roughening of the particlesurface may be achieved by acid treatment, e.g. by exposing tohydrochloric acid for several hours or even days (e.g. at least 10hours). Alternative treatment processes include the kraft process (usingsodium hydroxide and sodium sulfide); the soda process (“soda pumping”using sodium hydroxide); steam explosion processes, which may use acidicsteam; and enzymatic decomposition or by degradation by microorganisms.With wood or wood derived particles, these processes remove some of thelignin found in the cell wall and make holes, this exposes the cellulosefibres. This makes the particles themselves available to be degraded bythe preferred microorganisms in the formulation resulting in furtherroughening of their surface in the reservoir. In this way tendency toform a plug can be enhanced while the formulation is within thereservoir and after it has reached the zone of interest. Excessivedegradation of the particles can be controlled by cutting off the supplyof other essential nutrients to the microorganisms, other nutrients canotherwise be supplied in the injection liquid.

Injection liquid is usually fresh water or salt water that is injectedinto the reservoir through an injection well. Salt water can includefresh water to which salts are added, a mixture of sea water and freshwater, natural brackish water and undiluted sea water. The injectionliquid can be degassed, supplemented with biocides or exposed toradiation in order to reduce the number of microorganisms in theinjection liquid. Preferably the injection liquid is less saline thansea water, the salinity may be 3.5-6%, or less than 4%, possibly lessthan 3%. Injection liquid may contain nutrients, e.g. a growth mediumfor the microorganisms.

The growth medium in the formulation of the invention is intended tosupport the growth of the microorganisms within the formulation,although an adequate carbon source may be provided by the viscosifier.It can contain one or more suitable nitrogen sources, phosphorussources, potassium sources and trace element sources and vitamins, suchas are known in the field. Suitable components of the growth medium,including salts and minerals, are described in the Examples, inparticular in Freier medium which represents a suitable growth medium.Further qualities of growth medium can be added to the injection liquidin order to supply the microorganisms in the formulation with additionalnutrients. Nutrients may include further sources of carbon butpreferably will not.

The viscosifier may provide the necessary physical matrix to hold theformulation of the invention together, i.e. act as a binder or carrierfor the solid particles and microorganisms, allowing the slug to remainintact as it transports the particles. The use of viscosifiers, inparticular cellulose based viscosifiers, is well known to the skilledman, for example to enhance movement of drilling cuttings by aslurrification process. In some embodiments, the formulation may alsocomprise a cross-linker which helps to hold the solid particles andbacteria together and eases flow of the formulation to the pluggingsite. The cross-linker may also be degradeable by the bacteria and willtypically cross-link molecules of the viscosifier. Alternatively,molecules of the viscosifier, such as polysaccharide chains e.g. CMCchains, may be cross-linked without the need for an additionalcross-linker component as such. Cross-linking may be achieved, forexample, using a polyamine, chloromethypyridylium iodide or byirradiation. Suitable methods for cross-linking cellulose and cellulosederivatives are described in U.S. Pat. Nos. 5,304,620 and 6,734,298.

The viscosifier is typically either soluble in water or forms asuspension therein. On a weight by weight basis, the viscosifier may bepresent in solution/suspension (typically with water or brine) at150-10000 ppm, more usually 300-5000 ppm.

Viscosifiers are preferably polysaccharides and cellulose andderivatives thereof are especially preferred. Such derivatives includepolymers of glucopyranose monomers in which some or all (usually some)of the hydroxyl groups —OH have been substituted by groups —OR in whichR may be, for example, an alkyl, hydroxyalkyl or carboxylic acid moiety,typically the R groups are C₁₋₅ groups with C₁₋₃ being preferred.Derivatives incorporating carboxylic acid moieties may exist in saltform, e.g. sodium carboxymethyl cellulose. Preferred examples ofcellulose derivatives include carboxymethyl cellulose (CMC), polyanioniccelluloses generally, methyl cellulose or hydroxypropyl cellulose,hydroxyethyl cellulose, ethyl cellulose, nitrocellulose, hydroxyethylmethylcellulose and hydroxypropyl methylcellulose. With CMC and othersimilar cellulose derivatives, typically no additional cross-linker isrequired.

Cellulose derivatives such as CMC and other polyanionic celluloses varyin chain length and degree of substitution of the glucopyranosemonomers. CMC molecules are somewhat shorter, on average, than nativecellulose with uneven derivatization giving areas of high and lowsubstitution. This substitution is mostly 2-O- and 6-O-linked, but mayalso be 2,6-di-O-, 3-O-, 3,6-di-O-, 2,3-di-O- or 2,3,6-tri-O-.linked. Itappears that the substitution process is a slightly cooperative (withinresidues) rather than random process, giving slightly higher thanexpected unsubstituted and trisubstituted areas. CMC molecules are mostextended (rod-like) at low concentrations but at higher concentrationsthe molecules overlap and coil up and then, at high concentrations,entangle to become a thermoreversible gel. Increasing ionic strength andreducing pH both decrease the viscosity as they cause the polymer tobecome more coiled.

Suitable polyanionic cellulose viscosifiers have the CAS number9004-32-4, e.g. as supplied by Kelco Oil Field Group under the tradenames CELPOL®R, CELPOL®SLX and CELPOL®RX.

A further preferred cellulose polymer is Exilva, a microbifrillatedcellulose which has higher than typical numbers of accessible hydroxylgroups resulting in high water retention capability. This product formsa suspension in water and it is advisable to mix thoroughly afterdilution to ensure the fibrils are well separated and the product doesnot settle. A water-soluble polymer such as CMC, xanthan gum or guar maybe employed to stabilise the Exilva suspension. This product isavailable from Borregaard, Norway.

Other suitable polysaccharide viscosifiers are gums, such as areproduced by microorganisms, e.g. diutan gum and xanthan gum. Suitablegums are identified by their CAS numbers, CAS 11138-66-2, 125005-87-0and 595585-15-2, e.g. GEOVIS®XT and XANVIS® supplied by the Kelco OilField Group. Suitable bacteria to degrade xanthan and other gums areknown in the art, e.g. Peanibacillus alginolyticus XL-1,Verrucomicrobium sp. GD, Bacillus sp. GL1, Bacillus sp. YJ1,Cornynebacterium. Xanthan may first be degraded to acetate andpropionate, sulphate reducing bacteria (SRB) may then be employed tofurther metabolise these xanthan breakdown products.

As previously mentioned, preferred microorganisms in the formulation candegrade cellulose derivatives to form disaccharides and eventuallyhydrogen, carbon dioxide, ethanol, acetate and lactate. It isdegradation of the viscosifier which causes the shift from a flowablesuspension to a solid plug of packed particles. This relationshipbetween the enzymatic capabilities of the microorganism and theviscosifier is central to the present invention.

Freier-medium is a particularly suitable source for providing the saltsand minerals of the growth medium. The viscosifier, e.g. CMC, is thesole or main carbon source in the formulation. The growth medium(together with the viscosifier) serves to support microbial growth forlong enough to achieve breakdown of the viscosifier and conversion fromflowable suspension (slug) to solid plug. Freier-medium but with CMC asthe carbon source provides a suitable viscosifier plus growth mediummixture.

The viscosifier must be suitable as a viscosifier in a hydrocarbonreservoir environment and act as a carbon source for the microorganisms.The role of viscosifiers is well known in the field of oil recovery andhere this component must, in particular, act as a lubricant allowing theformulation in the form of a slug to move through the reservoir. Whenpresent the viscosifier provides a high viscosity environment but theformulation is mobile within the reservoir. When it has been degradedthe viscosity of the formulation is reduced (e.g. to a viscosity similarto water) but friction between the particles themselves and between theparticles and the matrix walls is increased such that plugging can takeplace.

The viscosifier will preferably provide the formulation with a viscosityof at least 2 cP, e.g. 5-15 cP, preferably about 10 cP when it isintroduced downhole but may be higher, e.g. at least 5 or 10 cP,possibly more than 15 or 20 cP. At this viscosity the formulation cantransport the particles, microorganisms and any additional growth mediathrough the fractures. When the microorganisms have broken down theviscosifier, the viscosity of the formulation decreases, for example toless than 3 or 2 cP, preferably about 1 cP; this will result in theformation of a solid plug (water has a viscosity of 1 cP at 20° C.).

In a further aspect, the invention provides a plug formed in ahydrocarbon reservoir, said plug comprising solid particles as definedherein and optionally microorganisms as defined herein, optionally inthe form of spores.

As an alternative to microorganisms in the formulation, it may bedesirable to use enzymes which are capable of digesting the viscosifier.The enzymes are typically saccharolytic or lignocellulolytic enzymes,preferably enzymes which can digest cellulose, including cellulosederivatives such as carboxymethyl cellulose. Such enzymes are knowngenerally as cellulases and are capable of hydrolysing the1,4-beta-D-glycosidic linkage found, inter alia, in cellulose andhemicellulose. The enzymes may tolerate different conditions as comparedto the microorganisms, in particular they may tolerate highertemperatures which may make them preferred for certain reservoirs. Theymay also be active at more extremes of salinity, pressure etc.

Thus, in a further aspect, the present invention provides a formulationcomprising:

-   -   (a) a cell-free enzyme;    -   (b) solid particles, preferably made from wood or other        cellulosic material; and    -   (c) a viscosifier which is a substrate for the cell free enzyme        of (a).

In a further aspect the present invention provides a formulationcomprising:

-   -   (a) microorganisms or cell-free enzymes;    -   (b) solid particles made from wood or a wood derived product;        and    -   (c) a viscosifier which is a substrate for the microorganisms or        cell-free enzymes of (a).

In the above formulations, the enzyme(s) is present in purified orpartially purified form, in other words not associated with living cells(not intracellular or bound to an intact cell membrane or cell wall).

The particles and viscosifier are as discussed above in the context offormulations comprising microorganisms.

Suitable enzymes are produced by fungi, bacteria and protozoans and theenzymes may be from or derived from bacteria which are themselves of usein formulations of the invention, e.g. Clostridium thermocellum,preferably the strain JW20. In preferred embodiments more than 1 enzymewill be included in the formulation, e.g. 2-4 enzymes that act todegrade cellulosic material.

In some embodiments a cellulase may be provided as part of amulti-enzyme complex known as a cellulosome. Cellulosomes are producedby many cellulolytic microorganisms, e.g. C. thermocellum, C.cellulolyticum and C. cellulovorans.

Suitable cellulases which may be used individually, together or as partof a cellulosome include: endocellulases, exocellulases orcellobiohydrolases, cellobiases or beta-glucosidases, oxidativecellulases or cellulose phosphorylases. Examples includeendo-1,4-beta-D-glucanase (beta-1,4-glucanase, beta-1,4-endoglucanhydrolase, endoglucanase D, 1,4-(1,3,1,4)-beta-D-glucan4-glucanohydrolase), carboxymethyl cellulase (CMCase), avicelase,celludextrinase, cellulase A, cellulosin AP, alkali cellulase, celluloseA 3, 9.5 cellulase, and pancellase SS. A CMCase is particularlypreferred.

Suitable enzymes will be catalytic in downhole conditions, in particularthe enzymes themselves can be considered thermophilic, or evenhyperthermophilic. For example tolerating temperatures in excess of 70°C., preferably in excess of 80 or 90° C., optionally in excess of 100°C. Naturally occurring enzymes which have been modified to adapt them todownhole conditions may be used.

In a further aspect, the present invention provides a method of treatinga hydrocarbon reservoir in order to prepare it for recovery ofhydrocarbons therefrom, which method comprises introducing into thereservoir a formulation of the invention.

Any hydrocarbon reservoir may be treated according to the presentinvention but carbonate reservoirs are preferred. Oil containingreservoirs are also preferred.

Alternatively viewed, the present invention provides a method ofestablishing a plug in a hydrocarbon reservoir, the method comprisingintroducing into the reservoir a formulation of the invention. Thepresent invention provides a method of establishing a plug in ahydrocarbon reservoir, the method comprising introducing into thereservoir a formulation comprising:

-   -   (a) microorganisms or cell-free enzymes;    -   (b) solid particles; and    -   (c) a viscosifier which is a substrate for the microorganisms or        cell-free enzymes of (a).

These methods may further comprise introducing injection liquid into thereservoir in order to push the formulation to a target region within thereservoir. Preferred features, definitions and so on described above inrelation to the solid particles, microorganisms and viscosifier of theformulations of the invention apply also to these methods and componentsemployed therein.

The reservoir is typically one that has previously producedhydrocarbons, and in particular will already have undergone a phase ofEnhanced Oil Recovery (EOR). It may be a carbonate or sandstonereservoir. These methods will typically be performed on reservoirs whichhave already been flooded with an injection liquid, e.g. water.Injection liquid will also generally be added after the introduction ofthe formulation in order to push the flowable formulation into thereservoir. The injection liquid may contain nutrients for the bacteriawithin the formulation, e.g. phosphates and other salts, vitamins andminerals and optionally further sources of carbon for the bacteria, e.g.cellulose, CMC or cellobiose.

In preferred embodiments plugging is achieved without having tointroduce additional nutrients, in particular a carbon source, for themicroorganisms. In other words, it is not necessary to maintain the plugby providing an on-going supply of nutrients, because the plugging isachieved by the solid particles, not biomass.

Thus the methods of the invention preferably comprise the followingsteps:

(i) introducing into the reservoir a formulation of the invention; and(ii) introducing injection liquid into the reservoir in order to pushthe formulation to a target region within the reservoir.

In a typical reservoir set up, injection liquid flows from the injectionwell, through the reservoir and exits (or partially exits) through theproduction well. Of course, the desire is that the injection liquid willalso force oil from the reservoir to the production well.

The target region within the reservoir is the region where it is desiredto form a plug. This may be a specific area, e.g. at a certain distancewithin the flow channels, where modelling or other studies have takenplace but need not be and the target region may only be understood invery general terms. The pressure generated by injection of liquid forcesthe formulation, which may be in the form of a suspension, into thereservoir along flow channels, e.g. fractures in a carbonate reservoir.As the viscosifier is degraded, the formulation develops a reducedability to flow and this resistance builds until the flowchannel/fracture is blocked; the particles are forced in the flowdirection and a plug forms of packed particles.

Generally the plugging process will block the larger, dominant fracturesor channels first (the most dominant waterways transport the largestvolume of water and will thus carry the dominant slug of particles) andthis would result in a new flooding pattern where new fractures orchannels became dominant, this process could take about 1 to 2 weeks.Then the process could be repeated to target the new dominant fracturesor channels. The process could be repeated several times, as required,in order to cause sufficient blocking to increase oil production.

Generally the larger the fracture the larger the particle size requiredfor effective blocking, e.g. a 13 mm wide fracture may be mosteffectively blocked using particles of 0.5 mm diameter and so as theprocess is repeated, the size of the particles introduced will typicallybe reduced with successive applications. In general, the diameter of theparticles will be 1-10% of the width of the fracture, preferably 3-8%,but may be greater, e.g. up to 30 or 20% of the width.

By the introduction of plugs (e.g. plugs in dominant and less dominantfractures or channels) in the matrix, it is possible to force injectionliquid into new zones and therefore not just reverse the decline insweep efficiency but change the shape of the sweep zone to access andrecover oil from new areas.

Through understanding of flow rate through the reservoir from injectionwell to production well, the positioning of the plug may be controlled.The change from flowable slug to solid plug may be initiated or enhancedby shutting off the injection of liquid or increasing or decreasing theflow thereof.

As has been described herein, key to the present invention is thetransition from a mobile liquid in which a viscosifier carries solidparticles to a pluggable mass of solid particles. This transition occursthrough degradation of the viscosifier which, as discussed herein, maybe achieved by microorganisms or enzymatic degradation. However, theprinciple is more general and thus, in a further aspect, the presentinvention provides a method of establishing a plug in a hydrocarbonreservoir, the method comprising introducing into the reservoir aformulation comprising a viscosifier as defined herein and solidparticles as defined herein and then reducing the viscosity of saidviscosifier, thereby causing said solid particles to form a plug withinsaid hydrocarbon reservoir. Unless otherwise clear in context, preferredand additional features of the earlier described aspects of theinvention also apply to this aspect. For example, this aspect of theinvention may conveniently involve a step of introducing injectionliquid into the reservoir in order to push the formulation to a targetregion within the reservoir prior to reducing the viscosity of theviscosifier. Viscosity is typically reduced by degradation of theviscosifier, that is conversion from a high molecular weight polymer tolow molecular weight molecules which have a viscosity similar to waterand are no longer able to prevent packing of the solid particles to forma plug. As in all aspects, when introduced into the reservoir theviscosifier is able to transport the solid particles, but once degraded,flow of the solid particles ceases and plugging takes place.

Without wishing to be bound by theory, it is believed that degradationof the viscosifier will begin in the front section of the slug and soplugging starts at the front end. At this stage the slug/plug is stillpermeable to water but particles are accumulating and condensing fromthe front and extending backwards causing an increase in backpressure.The backpressure forces the rear of the slug against the more solidfront portion. Over time the slug transforms into a mass of particlesand is no longer significantly water permeable. The particles mayconstitute about 40-45% of the total volume of the slug and so thelength of the slug will decrease as the viscosifier is degraded and acompacted plug forms. The plug then functions as a check valve.

The purpose of the plug is to allow a pressure differential to build upacross it, thereby forcing injection liquid out of the flow channel andinto the matrix, forcing oil out of the matrix. The plugs according tothe present invention may withstand a pressure differential of at least2000 PSI, preferably at least 4000 PSI, more preferably at least 6000PSI. As discussed elsewhere herein, the longer the plug in the floodingdirection, the higher the pressure differential it can withstand. Thisrelationship is approximately linear. By way of example, the pressure onthe injection side of the plug may be around 14,000 PSI and the pressureon the production side of the plug may be around 6,000 PSI.

Plug lengths may be measured in meters, preferred plugs being greaterthan 5 m, preferably greater than 10 m, more preferably greater than 25m in length, e.g. 50-100 meters. Generally speaking smaller particlesare best suited for longer plug lengths, e.g. a diameter ≤0.5 mm,preferably ≤0.2 mm for plug lengths greater than 1.5 m. The distancebetween the walls of the fracture to be plugged is also relevant, thusif a plug of 10 m or larger is desired, solid particles in theformulation should be 1000 times smaller than the width of the fracture.

The present invention provides a method of oil recovery from ahydrocarbon reservoir, which method comprises introducing injectionliquid into said reservoir, the reservoir comprising a plug of solidparticles as defined herein, and recovering oil from said reservoir.

The present invention also provides a method of oil recovery from ahydrocarbon reservoir, which method comprises establishing a plug insaid reservoir by performing such a method of plug establishment asdescribed herein, introducing injection liquid into said reservoir andrecovering oil from said reservoir. As mentioned earlier, injectionliquid is used to force oil from the reservoir to the production welland plugging established flow channels can enhance oil recovery byforcing injection liquid into new areas.

The formulation of the invention may conveniently be prepared by mixingthe components defined above; thus mixing microorganisms, optionally inspore form, particles and viscosifier, optionally together with across-linker and/or growth medium. Water may be added to the mixtureand/or the viscosifier may be blended with water and the othercomponents added thereto. Such a method of preparation constitutes afurther aspect of the present invention. Mixing of the solid particlesinto the liquid components may be by slurrification to form asuspension. Temperature and pH of the prepared formulation may becontrolled, in particular for transportation of the formulation where itis desired to inhibit degradation of the viscosifier by themicroorganisms.

In preferred embodiments, in order to reduce the buoyancy of particlesmade of wood or similar cellulosic materials, the particles or thematerial from which they are formed are submerged in order to saturatethem. Saturated wood or wood derived particles are preferred. Thisresults in an increase in the density of the particles to be closer tothe density of water.

The invention is further described in the following non-limitingExamples and the figures in which:

FIG. 1—is a drawing showing how the formation of dominant fracturesbetween Injector and Producer holes result in reduced Sweep Efficiencythrough a matrix.

FIG. 2—shows the set-up of experiments performed in a modelled chalkfracture. These experiments are outlined in Example 11 and demonstratedincreased pressure produced in the chalk fracture as a consequence ofthe formation of a plug of wooden particles. The experiments alsodemonstrated facilitated movement of wooden particles through the chalkfracture when said particles were suspended in a viscosifier (xanthan).

FIG. 3—shows graphs describing pressure (mbar) vs rate of flow of water(ml/min) through the model chalk fracture in the absence of a plug (topgraph) and with a plug formed of 1 mm diameter round wooden particles(bottom graph). A mobility reduction factor (MRF) of 861 was achievedupon formation of the wooden plug.

FIG. 4—is a graph showing the reduction of xanthan viscosity over thecourse of three days when xanthan is incubated at 30° C. in anoxicconditions with an anaerobic xanthan degrading bacteria. A concomitantincrease in bacterial cell growth is observed as the xanthan isdegraded.

FIG. 5—shows that the turbidity of Exilva (microfibrillated cellulose)is decreased (correlating with decreased viscosity) when incubated withClostridium thermocellum (CT). Bottles from left—right show Exilva+CT,Exilva (settled) and Exilva dispersed.

FIG. 6—is a schematic representation of apparatus for a two componentplug experiment (described in Example 14). The top image is a schematicdrawing of the particle and solvent (viscosifier) inlet. This inlet setup allows the viscosifier to wooden particle paste ratio to be adjustedeasily during the course of the experiment. The bottom image is aschematic diagram of the experimental set up for investigating theformation of a two component plug consisting of larger particlesinterspersed with smaller particles. The smaller particles may beintroduced at the same time as the larger particles or as a slug ofsecondary particles.

FIG. 7—is a graph showing pressure (mbar) over time (s) at a flow rateof 20 ml/min for a 2 component plug made up of 1 mm diameter particlesand ≤0.2 mm diameter particles. The performed test indicated that thetwo component plug tested could withstand a pressure of 11400 mbar andabove.

EXAMPLES Example 1—Degradation of Carbon/Methyl Cellulose by Clostridiumthermocellum Bacteria

Clostridium thermocellum (CT) JW20; ATCC 31549

Growth Medium

CT were cultured according to the methodology described by Freier et al.in Applied and Environmental Microbiology [1988] vol 54, No. 1, p204-211 but with carbon/methyl cellulose (CMC) present as the carbonsource. The CMC product used was CELPOL®RX, a highly viscous polyanioniccellulose (CAS number 9004-32-4) available from Kelco Oil Field Group.

Specifically, the culture medium contained (per liter of deionizedwater)

1.5 g KH₂PO₄4.2 g Na₂HPO₄. 12 H₂O

0.5 g NH₄Cl

0.5 g (NH₄)₂SO₄0.09 g MgCl₂. 6H₂O

0.03 g CaCl₂ 0.5 g NaHCO₃

2 g of yeast extract0.5 ml of vitamin solution. The vitamin solution contained (per liter ofdistilled water) 40 mg of biotin, 100 mg of p-aminobenzoic acid, 40 mgof folic acid, 100 mg of pantothenic acid calcium salt, 100 mg ofnicotinic acid, 2 mg of vitamin B12, 100 mg of thiamine hydrochloride,200 mg of pyridoxine hydrochloride, 100 mg of thioctic acid, and 10 mgof riboflavin.

5 ml of mineral solution. The mineral solution contained (per liter ofdistilled water) 1.5 g of nitriloacetic acid, 3 g of MgSO₄-7H₂0, 0.5 gof MnSO₄. H20, 1 g of NaCl, 0.1 g of FeSO₄ 7H20, 0.1 g of Co(NO₃)₂.6H₂0,0.1 g of CaCl₂ (anhydrous), 0.1 g of ZnSO₄. 7H₂0, 50 mg of NiCl₂, 10 mgof CuSO₄*5H₂0, 10 mg of AlK₂(SO₄)₃ (anhydrous), 10 mg of boric acid, 10mg of Na₂MoO₄*2H₂0, 10 mg of Na₂WO₄-2H₂0, and 1 mg of Na₂SeO₃(anhydrous)

1% carboxymethyl cellulose (CMC).

Growth Experiments

A culture of CT bacteria was inoculated and allowed to grow in a flaskcontaining the above growth medium as described in Freier et al. suprafor 5 days (referred to herein as Freier medium).

The Freier approach to CT culturing was modified with bottles containing3 different fractions of oil. One set of bottles contained 90% of oil,one set of bottles contained 50% of oil and the last set of bottlescontained 10% of oil. The remaining liquid contained the Freier mediumwith CMC at 1%. Bottles containing 100% Freier medium with CMC (1%) wereprovided as control. The bottles were shaken every 3 hours during theday. The bottles were opened after 1 week.

Ethanol was produced in concentrations corresponding to the volume andconcentration of growth medium containing CMC, indicating degradation ofCMC.

The oil did not have a inhibiting effect on the culture. The culturedoes not grow and metabolize within the oily fraction. We concluded thatthe growth medium was effectively removed in the high concentration ofoil due to the fact that oil and water are not soluble. We alsoconcluded that the culture could not utilize hydrocarbons as a carbonsource.

Viscosity Test

The media collected after the above degradation was added to a glasscylinder and the time taken for a lead ball to sink through the liquidwas measured. The experiment was repeated with pure water in place ofthe growth medium and with a sample of the growth medium which had notbeen inoculated with CT.

Media which had been exposed to CT as described above allowed the leadball to move through it (acceleration, time and maximum velocity) in asimilar fashion to the pure water. The media which had not beencontacted with CT, on the other hand, offered significant resistance tothe passage of the ball.

Using this test it was no longer possible to detect CMC in the samplewhich had been in contact with CT.

These experiments indicated that most CMC is degraded by CT and that theresultant viscosity is similar to that of pure water.

Example 2—Generation of Wooden Particles

Sandpaper (different grades of sandpaper result in different sizedparticles) was applied to pieces of wood (Corylus avellana) to generateparticles of about 200μ, 500μ or 1-2 mm in diameter.

Example 3—Plugging Experiments Using Wooden and Sand Particles

Wood particles of 0.5 mm and 1-2 mm in diameter produced as described inExample 2 were inserted with water into a transparent hose of 15 mmdiameter, 150 cm in length with a downstream valve. After filling thehose, a slowly increasing pressure differential was applied by injectingwater from one end of the hose to provide a backpressure of 8 bars. Theplug was pushed to become more compact and resisted the backpressureuntil the backpressure reached a maximum. The plug length could becontrolled by the volume of particles added and plug lengths of 10 and15 cm were generated. Before the maximum backpressure was reached theplug started moving in the flow direction, the particles stickingtogether and the plug gliding along the hose. Shaking the hose tended torelease the plug, enabling it to slide further. The larger (1-2 mm)particles allowed a greater flow of water through the plug.

The experiments were repeated with sand particles. These were much lesseffective at plugging and the particles did not stick together as theplug accelerated through the hose. When a 25 mm plug was generated thenthere was a plugging effect but it was very easy to release by shaking.

The results of these tests are summarised in Table 1 below.

TABLE 1 Max Particle Hose Plug Diff Size diam. size Pres. Quality Shape(mm) (mm) (mm) (bar) Observations Wood Rounded 0.5 15 10/15 8 “10”: Slippres. <8 bar particle “15”: Slip pres. >8 bar Maintained plug integrityWood Rounded 1-2 15 10/15 8 “10”: Slip pres. <8 bar particle “15”: Slippres. >8 bar Maintained plug integrity Sand Rounded <1 15 8 No plugintegrity up to 25 bar

Example 4—Acid Treatment of Particles

Particles generated as described in Example 2 were exposed toconcentrated hydrochloric acid for 15 hours and then an alkaline washwas used to increase the pH and establish a stable pH of about 7. Wateralone could be used to remove the acid.

The particles were added to Freier medium as described in Example 1 butwith no carbon source (CMC absent) and contacted with CT. The bacteriaattacked and partially degraded the particle.

The particles were studied under a microscope. The treated particleswere much more deformed and “hairy” or “fluffy” in appearance than thosewhich were not acid treated.

The cell walls of wood have lignin and cellulose. The above acidtreatment attacks the lignin layer making the cellulose parts accessibleto degradation by cellulolytic bacteria such as CT.

Example 5—Industrial Scale Production of Particles

Slurrification machinery (National Oil Well Slurrification Unit) used inoil drilling to process coarse cuttings can be used to process wood togenerate particles suitable for use in the present invention. The woodis run through a mill with water under high pressure and after about 10minutes the suspension is forced through a mesh of the desired size.These mesh filters effectively size the particles with the largerparticles which cannot pass through the filter being recycled forfurther milling. The resultant particles can be as small as 200μ indiameter. The particles are adequately uniform and solid but saturated.

Example 6—In Situ Set Up Features of Exemplary Carbonate HydrocarbonReservoir:

Volume of fracture 100 barrels (1 m³=6.29 barrels)Length from injection well to production well: 2000 feetVolume of injection liquid: 20000 barrels per dayFlow rate: 30 minDifferential pressure max: 6000 PSIExpected width in fractured structure: 1-5 mm

The flow rate is the time taken for liquid to pass through the reservoirfrom injection well to production well. This example reservoir which maybe treated according to the invention exhibits an extreme flow rateindicative of an extensive system of well developed fractures.

The differential pressure maximum is the maximum pressure differentialthat it is desired generate across the plug.

If required the fractures can be pre-treated by hydrochloric acid toincrease the resistance of the fracture walls, i.e. to increase thepotential for friction.

The suspension of particles, bacteria etc. is injected into theinjection well system. This is hydraulically forced further into thefracture system by back pressuring with injection water. The suspensionwill displace all alkaline water.

The bacteria attack any pure cellulose and the carboxymethyl cellulosewithin the suspension. The cellulose inside the particles is onlydegraded if the particles are pre-treated for the purpose.

Example 7—Further Tests to Study Plugging Effects in Sandpacks and HosesCeramic Particles:

Long transparent hoses of 13 mm diameter were used as equivalents to amagnified connected pore system. Particles of different sizes whereflooded through the hoses as slugs to observe if the particles were ableto form plugs. The particles were of same size and same shape and wereof solid ceramics. The volume of particles introduced was equivalent toan 8 cm long plug. The test system was capable of supporting abackpressure to the plug of 15 Bars.

Particles of the following size and shape were tested:

1. Ø<0.5 mm

2. Ø=1-2 mm

3. Ø>5 mm

Observations:

No plugs formed.

Wooden Particles:

The same test was performed with water—soaked particles of the sameshape size and shape made of wood—Spruce. No formation of plug wasobserved of particles sized Ø<0.5 mm and Ø>5 mm. However plugs wereformed by use of particles sized Ø=1-2 mm. The plug was flushed out ofthe hose by a backpressure exceeding 8 bars.

All the tests including the non-plugging tests were repeated 10 timesand showed the same results.

Test to Understand the Mechanisms by which the Wooden Particles Plugged:

Similar test setups as described above were performed using the woodenparticles. The structure of the particles were studied through thetransparent hose showing that the particles deformed slightly, they wereoriginally round shaped and soaked by water.

The backpressure led to the wooden particles forming an oval shape witha slightly soft surface.

Comparing the surface of ceramic particles to the surface of the woodparticles clearly revealed the potential for higher surface friction onthe wood particles. Studying the surface of the wood particles with amicroscope revealed small fiber threads more or less as hair on thesurface of the particle, while the ceramic particle was smooth.

Thus effective plugging is dependent on the particle diameter (relativeto the diameter of the hose/fracture), deformability and the surfacefriction of the particles.

Sandpack Test

A sandpack, 10 cm in length, 5 cm in diameter containing grains of 1 mmdiameter was set up and soaked wooden particles of 0.05 mm diameter wereadded. The particle size was selected in correlation to the relativesize of the pore mouth. Pores are formed as a series of interconnectingvoids between the particles, the size of the pores and thus the poremouth being dependent on the size/diameter of the grains. The system wasfirst flooded with water and after flooding, particles were added to thewater. The particles blocked the pores immediately, i.e. did not enterthe pack and form a plug.

A further test was performed using a sandpack of larger diameter andpebbles of 10-17 mm in size. These pebbles formed an enlarged poresystem generating a channel through the sandpack. Sand was packed aroundthe pebbles to provide a single channel through the sandpack.

Flooding was initiated with a viscosity 10 cP and the particles floodedthrough the system. A series of floodings with different viscositieswere performed (reduced by 2 cP per flooding). The system started toplug when flooded with viscosity below 4 cP. The agent used to controlviscosity was Carboxyl Methyl Cellulose (CMC) dissolved in the water.

Example 8—Testing Viscosity Reduction

Clostridium thermocellum JW20 represents an example bacteria which hasenzymatic capabilities to degrade Carboxyl Methyl Cellulose CMC and PolyAnionic Cellulose (PAC). A product of CMC was used to viscosify thecarrier fluid. 5% was added to bring up viscosity of the water to 10 cP.Before adding CMC to the water a solution of nutrients equivalent to 1%Vol was added to the water. The nutrients is a defined composition basedon the Freier—Medium in which the cellobiose is replaced with CMC 1:1Vol %:Vol %.

Viscosity measurements shows that the viscosity of the fluid is alteredfrom 10 cP to 1.5 cP.

Example 9—Blocking Connected Pore Systems

A test was set up equivalent to the Sandpack test of Example 7 andflooded with the carrier fluid including modified Freier media with CMC,CT and wooden particles at 35% Vol of the liquid. The composition wasinjected into the sandpack and shut in for 2 weeks. The system wasflooded with pure water, the injection pressure had to be elevated to 12bars to resume flooding through the system. When opening the Sandpack itwas observed that the particles were blocking the connected poresystems.

The test demonstrates that it is possible to introduce particles intothe sandpack via the injection water, transport and permanently locatethem. This operation is possible where a viscous fluid can transport theparticles and where the viscosity can be reduced by microbial activitythereby aggregating particles in the pores. An operation of this kindreduces permeability dramatically in the sandpack.

Example 9—Investigating Plug Length

A test set up as first described in Example 7 was prepared using woodenparticles of 1-2 mm and the same results were observed. Then the pluglength was increased to 15 cm and a backpressure of 15 Bars was exceededbefore the plug was forced out of the hose (the test was repeated 6times with the same result).

Example 10—Testing a Different Shaped Hose

The test described in Example 7 was repeated using a 100 cm hose with a5 cm diameter that has been reshaped to be an oval (width 1.3 cm andheight 7.15 cm). Plugging was shown under the same circumstances as seenin Example 7.

Example 11—Chalk Fracture Experiment

An artificial fracture was created in natural Austin Chalk (see FIG. 2).Wooden particles of 1 mm diameter in water only (no viscosifier) wereflowed through the fracture and formed a plug immediately at the inletover the first 10 cm of the fracture.

Reduction of particle mobility and pressure build up upon plug formationin the chalk core was measured (see FIG. 3). The differential pressurein the fracture before the plug was generated was 0.00023 mbar and afterplug formation was 0.19803 mbar equalling a mobility reduction factor of861 (see FIG. 3).

In order to demonstrate the ability of a viscosifier to facilitatetransport of wooden particles to fracture sites distant from the site ofinjection, an experiment was performed in the same fractured chalkcomparing 1 mm diameter particles suspended in water and the viscosifierxanthan. The viscosity of the water containing the 1 mm wooden particleswas increased to 2000 centipoise by the addition of xanthan. When theresultant suspension was introduced into the chalk fracture theparticles were able to move through the fracture relatively unimpeded.This demonstrates the ability of a viscosifier to facilitate movement ofplugging particles to sites remote from the injection well in a chalkfracture.

Example 12—Buoyancy of Wooden Particles

In order to adjust the buoyancy of wooden particles for optimumsuspension at different viscosities, the particles may be soaked inwater or brine.

Wood particles are filled into a pressure cylinder containing brine.Pressure is increased at a rate of 2 bar/hour up to a desired pressure,typically between 2 and 20 bar. The particles are kept at the givenpressure for minimum 2 days, maximum 1 week. Pressure is then reduced toatmospheric pressure over a time period of 1 hour. The composition ofbrine, the absolute pressure and the pressure exposure period is variedto adjust wood particle density.

In one particular example, 50 g of wood particles were filled into a 200ml stainless steel pressure cylinder. Pressure was increased byinjecting brine at constant pressure step wise until reaching 20 bar.The pressure was increased at a rate of 2 bar/hour. Pressure wasmaintained by injection pump at 20 bar for 1 week. Pressure was thenreleased with a gradient of 20 bar/hour to atmospheric pressure.

Example 13

A) Degradation of Xanthan by Anaerobic Xanthan-Degrading Bacteria

A microbial system for degradation of the viscosity of a slug has beenestablished. The slug consists of a xanthan based biopolymer, anaerobicxanthan-degrading bacteria and surplus of mineral nutrients, traceelements, vitamins and nitrate. The microbes operated optimally atmesophilic conditions (20-30° C.) and sea water salinity. In a testsystem with 500 ppm xanthan biopolymer, a complete degradation ofviscosity was observed within 2 days (FIG. 4). The concomitant increasein cell number verifies that the biopolymer was utilized for anaerobicgrowth of the bacteria. The degradation time of the slug may beoptimized for different biopolymer concentrations by adjusting theinitial cell number and essential nutrients in the slug.

B) Degradation of Exilva (Cellulose) by Clostridium thermocellum

Clostridium thermocellum is able to degrade the microfibrillatedcellulose polymer product named Exilva. Exilva is visible in the growthmedium as a turbid phase at the start of incubation. As degradationoccurs, the turbidity decreases and finally leaves the growth mediumclear at end of the growth phase (FIG. 5).

Example 14—Two Component Plug

In certain circumstances plugging of fractures may be optimised by useof a two component plug comprising particles of a larger size incombination with smaller particles which are capable of filling the voidspace between the larger particles, thus decreasing permeability of theplug. Previous results showed that a plug consisting of the 1 mm roundwood particles gave a MRF value of 700-1100. However, the permeabilityof such plug may be further reduced by the injection of a second slug ofsmaller particles (smaller wood particles ≤0.2 mm in diameter (seived))which are introduced to fill the void space between the larger 1 mmround wood particles.

In order to demonstrate this principle the following experiment wasperformed.

A transparent tube of 0.6 cm in diameter and 50 cm length was used as alaboratory analogue to a fracture (see FIG. 6). Two differentialpressure transducers were placed at either end of the tube.

Initially the tube was filled by the larger primary particles, 1 mmround particles, transported into the tube in a viscous slug (see FIG.6). To enter the tube, particles had to pass through a 0.45 cm diameterfront restriction. A viscous slug was necessary to avoid plugging of thefront restriction; xanthan (700 cP@10.1/s) was used for this purpose.The particles did not pass through the end restriction. The filling ofprimary particles was performed by gravity drainage. The plug lengthformed was about 24.5 cm at the end of the tube. A MRF of 700 wasrecorded using the primary particles only after flooding of the systemwith 500 ppm xanthan diluted in brine with a viscosity of 28 cP@10, 1/s.

The secondary particles, ≤0.2 mm diameter wood particles (7.2 wt %) wereinjected by a viscous slug consisting of 500 ppm xanthan. To obtain ahomogeneous slug of secondary particles the injection was performed byco-injection of solvent (viscosifier) and wood chip paste and an inlinemixer was used to combine the particles with the viscosifier (see FIG.6, top image). Co-injection of the separate components of the slug ispractical for adjusting the particle to viscosifier ratio during theexperiments.

The introduction of secondary particles plugged in the first part (12.1cm) of the plug measured by the DP1. The calculated MRF value, comparedto the primary particle plug, was 728 for the first part of the plug(0-12.1 cm), demonstrating considerably decreased permeability comparedto the primary plug alone

FIG. 7 is a graph showing pressure (mbar) over time (s) at a flow rateof 20 ml/min for the 2 component plug made up of 1 mm particles and ≤0.2mm particles. The performed test indicated that the two component plugcan withstand a pressure of 11400 mbar and above.

1-21. (canceled)
 22. A formulation comprising: (a) microorganisms or cell-free enzymes; (b) solid particles; and (c) a viscosifier which is a substrate for the microorganisms or cell-free enzymes of (a); wherein the solid particles (i) are substantially spherical, and (ii) have a core which is not deformable and an outer layer which is deformable.
 23. The formulation of claim 22, further comprising growth medium.
 24. The formulation of claim 22, wherein the microorganisms or cell-free enzymes are saccharolytic or lignocellulolytic.
 25. The formulation as claimed in claim 22, wherein the microorganisms are, or wherein the cell-free enzymes are from, Clostridium thermocellum or Acidothermus cellulolyticus.
 26. The formulation of claim 22, wherein the viscosifier comprises, cellulose, hemicellulose or a derivative thereof or a polysaccharide gum.
 27. The formulation of claim 26, wherein the viscosifier comprises a polyanionic cellulose or a microfibrillated cellulose.
 28. The formulation of claim 27, wherein the viscosifier comprises carboxymethyl cellulose.
 29. The formulation of claim 22, wherein the solid particles have the same density as the rest of the formulation.
 30. The formulation of claim 22, wherein the solid particles are substantially uniform in size.
 31. The formulation of claim 22, wherein the formulation is made up of 25-70% by volume of the solid particles.
 32. The formulation of claim 22, wherein the solid particles of the formulation comprise a first population of solid particles and a second population of solid particles, wherein said first population are at least five times larger than said second population.
 33. The formulation of claim 22, wherein the solid particles of the formulation comprise a population of solid particles which are 0.05 to 5 mm in diameter.
 34. The formulation of claim 22, wherein the solid particles of the formulation comprise a population of solid particles which are <0.2 mm but ≥1 μm in diameter.
 35. The formulation as claimed in claim 22, wherein the solid particles of the formulation comprise a population of solid particles which are 1 to 100 μm in diameter.
 36. The formulation of claim 22, wherein the formulation comprises a colloid comprising a continuous phase and a dispersed phase in which the solid particles are the dispersed phase.
 37. The formulation of claim 22, wherein the formulation is an aqueous formulation.
 38. The formulation of claim 22, wherein the formulation has a viscosity of 5-15 cPa.
 39. The formulation of claim 22, wherein the particles are made from wood or a wood derived product. 